Reverse DNS (PTR)
Find the hostname mapped to any IP address.
Key terms, defined
- PTR record
- DNS record type that maps an IP address to a canonical hostname. Lives in the in-addr.arpa or ip6.arpa zones.
- FCrDNS
- Forward-Confirmed reverse DNS. Validates that the PTR hostname for an IP, when resolved back, returns that same IP. Required by major mail providers.
- in-addr.arpa
- Special reverse-mapping DNS zone for IPv4. An address 8.8.8.8 is queried as 8.8.8.8.in-addr.arpa.
- ip6.arpa
- IPv6 equivalent of in-addr.arpa. Uses nibble-reversed hex notation for the queried address.
- rDNS delegation
- Authority over an IP block's reverse zone, granted by the Regional Internet Registry (ARIN, RIPE, APNIC, etc.) to the address holder.
- Generic rDNS
- Provider-default PTR like ec2-52-1-2-3.compute-1.amazonaws.com. Functional but signals shared infrastructure to anti-abuse systems.
Frequently asked questions
Definitions and answers about reverse DNS, PTR records, and FCrDNS — the signals that govern email deliverability and IP-based trust.
What is a reverse DNS (PTR) record?+
A PTR record maps an IP address back to a hostname — the inverse of a normal forward A/AAAA record. It is published in the in-addr.arpa (IPv4) or ip6.arpa (IPv6) zone by whoever controls the IP block, typically the hosting provider or ISP.
Why does reverse DNS matter for email deliverability?+
Most production mail servers reject or score down inbound mail from IPs without matching PTR records. Forward-confirmed reverse DNS (FCrDNS) — where the PTR hostname resolves back to the same IP — is required by many large mailbox providers, including Google and Microsoft.
Who controls the PTR record for an IP?+
The PTR record is delegated by the IP block owner, not the domain owner. For a leased VPS or cloud IP, the hosting provider (AWS, DigitalOcean, Hetzner, etc.) sets it — usually through their control panel. You cannot set a PTR for an IP you do not control.
Why does an IP have no PTR record?+
Residential, mobile, or freshly provisioned IPs often have no PTR set, or a generic provider-default. This is normal but signals consumer connectivity to anti-spam systems, which is why home connections cannot reliably send transactional email.
Can one IP have multiple PTR records?+
Technically yes, but it is discouraged. Multiple PTRs confuse FCrDNS validation and reverse-lookup tooling. Best practice is exactly one PTR per IP, matching the primary hostname.